Field of the Invention
The invention relates generally to the field of nuclear magnetic resonance tools. More specifically, the invention relates to logging-while-drilling nuclear magnetic resonance tools having magnets external to the drill collar and magnetically permeable members to control the magnetic field gradient.
Background Art
Nuclear magnetic resonance (NMR) can be used to determine various characteristics of subsurface formations and/or samples. NMR logging tools can be used downhole to obtain these characteristics, which then can be used to assist in the determination of, for example, the presence, absence, and/or location of hydrocarbons in a given formation or sample.
Conventional NMR logging, well known in the art, generally involves deploying in a wellbore an NMR instrument, which uses magnetic fields to generate and detect various RF signals from nuclei in a formation or sample. Certain example NMR techniques are described in U.S. Pat. No. 6,232,778 assigned to Schlumberger Technology Corp., the entire disclosure of which is hereby incorporated by reference.
NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field B0 to the formation, such as through the use of permanent magnets.
In conventional logging-while-drilling (LWD) NMR tools, these permanent magnets are generally placed within the drill collar, which provides a protective housing for the magnets and other components of the NMR tools. Such protection may be helpful to reduce the risk of damage from drilling, both in terms of shock and wear. Such conventional tools can involve the building of the magnets into a housing to provide a framework for the magnets to be attached thereto. This framework can decrease the volume of magnetic material that can be used. This is critical for NMR, as the Signal to Noise Ratio (SNR) changes as a function of magnetic field strength and the magnetic field gradient. Other disadvantages, such as cumbersome accessibility of the magnets and other NMR tool components, also exist with conventional LWD systems that place the NMR magnet assemblies within the drill collar.
Accordingly, there is a need in the art for methods and systems for obtaining NMR measurements that overcome one or more of the deficiencies that exist with conventional methods.